Utility for chaining commands and incrementally checking the results

pip install assertionchain==0.1.1



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An AssertionChain is a wrapper for a group of commands that must happen in sequence.


pip install assertionchain


  • Control order of execution of a group of functions
  • Terminate execution of the rest of the chain if certain conditions are not met

Simple example:

    .do(lambda: 1 + 1, 'Add two numbers together') \
    .expect(3, message='Step "{step}" did not result in expected value {expected}, value was: {actual}') \

This is roughly equivalent to:

val = 1 + 1
assert val == 3

The above example is certainly more readable, so why use an AssertionChain at all? For starters, the chain encourages the user to provide line-by-line detail about each step being executed. So if you want to understand why the above assertion failed, you would have to construct a detailed error message indicating the actual and expected value along with the step that produced the actual value. That's not particularly hard:

expected = 3
val = 1 + 1
message = 'Adding 1 + 1 did not result in expected value {expected}, value was: {actual}'.format(
assert val == expected, message

But having to repeat this every time you run a command can become very, very tedious. AssertionChain provides this API for performing incremental checks on each operation being executed in the chain, reducing the overhead of having to type all of this out.

It is a simple utility for grouping related actions and ensuring each step succeeds. For instance, assume we have a suite of functions that interact with files (get_contents, write_contents, delete_file). Their contract stipulates that each function will return True, False, or a value depending on whether or not they were successful. We can use an AssertionChain to make sure each step was successful:

filename = '/tmp/sd9x0c2'
new_contents = 'myfile'

# Write the file, retrieve the contents
contents = AssertionChain()\
    .do(lambda: get_contents(filename), 'Retrieve file contents').expect(False, operator='is not')\
    .do(lambda: write_contents(filename, new_contents), 'Write file content').expect(True, operator='is')\
    .do(lambda: write_contents(filename, ''), 'Write empty file content').expect(True, operator='is')\

# Delete the file
AssertionChain().do(lambda: delete_file(filename), 'Delete file').expect(True, operator='is').perform()